The present invention relates to a reinforced ceramic investment casting shell mold especially useful in the casting of large industrial gas turbine and aerospace components and a method of making same such that the shell mold exhibits increased strength and creep resistance at elevated casting temperatures to maintain casting dimensional control.
Ceramic investment shell molds are widely used in the investment casting of superalloys and other metals/alloys to produce gas turbine engine components, such as turbine blades, and aerospace components, such as structural airframe components, to near net shape where dimensional control of the casting is provided by the shell mold cavity dimensions.
The need for industrial gas turbines (IGT""s) with improved operating performance has increased the demand for large IGT components with directionally solidified (DS) microstructures, such as columnar grain and single crystal cast microstructures. However, production of DS components subjects the ceramic investment shell mold to casting parameters, such as elevated temperature, metallostatic pressure and time, beyond the capability of present ceramic investment shell molds. In particular, present ceramic investment shell molds are susceptible to bulging and cracking during DS casting processes, especially when the shell mold is filled with a large quantity of molten metal/alloy at higher casting temperature and longer times needed, for example, to effect directional soldification of the IGT components.
When the investment shell mold bulges or sags during the DS casting process, dimensional control is lost and inaccurately dimensioned cast components are produced. Moreover, a significant cracking of the shell mold can occur and result in runout of molten metal/alloy and a scrap casting.
The most common ceramic mold materials, such as alumina and zirconia, used to produce ceramic shell molds exhibit creep deformation at about 2700 degrees F. with the creep deformation increasing with increasing temperature and hold time at temperature. Hold times in excess of 3 hours and temperature in excess of 2800 degrees F. are common in the casting of large directionally solidified IGT components. These casting parameters together with increased metallostatic pressure involved are severe enough that conventional ceramic shell molds have not been suitable for the casting of large directionally solidified IGT components. In particular, use of conventional ceramic shell molds for the casting of large directionally solidified IGT blades has resulted in changes in the blade chord width or changes to blade bow and displacment indicative of mold bulging or sagging during DS casting.
Therefore, there is an acute need for more robust ceramic shell molds that can withstand these severe casting parameters and resist creep deformation, such as bulging and sagging, as well as cracking to enable casting of large directionally solidified IGT components with dimensional control.
Several attempts have been investigated to raise the capability of ceramic shell molds manufactured using conventional ceramic materials. For example, one attempt has involved use of composite shell molds made of combinations of ceramic materials to minmize grain growth and hence reduce creep deformation of the mold. U.S. Pat. Reissue No. 34,702 describes another attempt wherein alumina-based or mullite-based ceramic fibrous reinforcement is wrapped about the mold. These techniques, although having further pushed the limit of conventional shell molds, have been found not to be sufficient to meet the stringent casting parameters imposed in the casting of large directionally solidified IGT components with dimensional control.
An object of the present invention is to provide a ceramic investment shell mold reinforced in a manner to exhibit improved resistance to creep deformation and cracking at elevated casting temperatures, especially under the aforementioned severe casting parameters demanded by casting of large directionally solidified IGT components with dimensional control.
Another object of the present invention is to provide a method of making a ceramic investment shell mold reinforced in a manner to exhibit improved resistance to creep deformation and cracking at elevated casting temperatures.
Still another object of the present invention is to provide a method of casting large directionally solidified IGT components with dimensional control.
To achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a ceramic investment shell mold is reinforced with a carbon based fibrous reinforcement having an extremely high tensile strength sufficient to reduce creep deformation of the mold, such as bulging or sagging, at high casting temperature, especially at temperatures experienced during casting of large directionally solidified IGT components. Preferably, the carbon based fibrous reinforcement is made of carbon fibers or filaments having a tensile strength of at least about 250,000 psi at room temperature (70 degrees F.) and a coefficient of thermal expansion that is less than the average coefficient of thermal expansion of shell mold to provide compressive loading of the mold.
Carbon fiber cordage (comprising a large number of carbon fibers or filaments) having a cordage breaking strength of 90 to 165 pound force, preferably 120 to 165 pound force, at room temperature is especially preferred as the reinforcement.
The carbon based fibrous reinforcement preferably is disposed at the ceramic slurry/stucco layers forming the intermdiate thickness of the shell mold wall. For example only, the carbon based fibrous reinforcement can be disposed around the 6th to the 9th shell mold layers forming an intermediate thickness of the shell mold wall.
In a method embodiment of the present invention, a pattern having the desired shape of the cast component to be produced is dipped in ceramic slurry and then stuccoed with relatively coarse ceramic stucco with the sequence repeated to build up a shell mold wall comprising repeating ceramic slurry/stucco layers on the pattern. At intermediate ceramic slurry/stucco layers defining an intermediate shell mold wall thickness, the carbon based fibrous reinforcement is applied around the shell mold wall, preferably by wrapping in a sprial configuration about the intermediate shell mold wall, followed by continuation of the dipping and stuccoing steps to build up the overall shell mold wall thickness over the reinforcement. When used, the sprial wrapped carbon based fibrous reinforcement can have a space between successive wraps of about 0.2 to 1 inch.
A carbon based woven or braided fiber cloth like reinforcement can be used to reinforce regions of the shell mold which render difficult or prohibit wrapping of the reinforement around the shell mold.
A method of casting large directionally solidified IGT components with dimensional control in accordance with an embodiment of the present invention involves preheating a ceramic investment shell mold reinforced as decribed above to an elevated casting temperature above about 2750 degrees F., introducing molten metal into the preheated shell mold, and directionally solidifying the molten metal residing in the shell mold by propagating a solidification front through the molten metal over an extended time period to form a columnar grain or single crystal microstructure. Large IGT components typically involve introduction of molten metal in the range of about 40 to about 300 pounds molten metal into the preheated shell mold and solidified over a time period of about 3 to about 6 hours therein.